Mortality attributable to seasonal influenza A and B infections in Thailand, 2005-2009: a longitudinal study

Abstract

Influenza epidemiology differs substantially in tropical and temperate zones, but estimates of seasonal influenza mortality in developing countries in the tropics are lacking. We aimed to quantify mortality due to seasonal influenza in Thailand, a tropical middle-income country. Time series of polymerase chain reaction-confirmed influenza infections between 2005 and 2009 were constructed from a sentinel surveillance network. These were combined with influenza-like illness data to derive measures of influenza activity and relationships to mortality by using a Bayesian regression framework. We estimated 6.1 (95% credible interval: 0.5, 12.4) annual deaths per 100,000 population attributable to influenza A and B, predominantly in those aged ≥60 years, with the largest contribution from influenza A(H1N1) in 3 out of 4 years. For A(H3N2), the relationship between influenza activity and mortality varied over time. Influenza was associated with increases in deaths classified as resulting from respiratory disease (posterior probability of positive association, 99.8%), cancer (98.6%), renal disease (98.0%), and liver disease (99.2%). No association with circulatory disease mortality was found. Seasonal influenza infections are associated with substantial mortality in Thailand, but evidence for the strong relationship between influenza activity and circulatory disease mortality reported in temperate countries is lacking.

Keywords: Bayesian regression; burden; developing country; influenza; middle-income country; mortality; seasonal variation; tropics.

Figure 1.

Time series of observed influenza-like illness (ILI) and influenza activity measures (B₁, B₂, B₃) in Thailand, 2005–2009. Influenza activity measures represent the product of influenza-like illness and the percentage of tested isolates positive for a given type in each week. Broken lines show the proportion of tested isolates positive for influenza (A) and positive for specific influenza types (B–D).

Figure 2.

Observed and expected deaths by age group in Thailand, 2005–2009. The figure shows recorded weekly deaths in those aged ≤17 years (open circles), 18–59 years (squares), ≥60 years (triangles), all ages (diamonds), and the expected number of deaths predicted by the full model (red lines) and expected number of deaths excluding those due to influenza (black lines). Shaded areas indicate excess deaths attributed to influenza (pink if positive, blue if negative). Vertical dashed lines indicate the first week of the calendar year, and vertical dotted lines mark the Thai New Year (April 13–15).

Figure 3.

Observed and expected deaths attributed to causes commonly associated with influenza and 2 control causes in Thailand, 2005–2009. Shaded areas indicate excess deaths attributed to influenza (pink if positive, blue if negative). Vertical dashed lines indicate the first week of the calendar year, and vertical dotted lines mark the Thai New Year (April 13–15). Apr, April; Jan, January.